The physics of time travel
Want to write a movie about time travel not riddled with physics-breaking inconsistencies, mind-screwing paradoxes and critic-infuriating plot holes? Physicist Dave Goldberg lays down the rules in “Time-Traveling for Dummies“, in Slate Magazine.
In the article he reviews how upcoming movie The Time-Traveler’s Wife, a movie where the main character has a rare genetic disorder that causes him to time-travel unpredictably, obeys the rules of time-traveling.
Supposing you’ve inherited a time machine from your grandfather. Presumably, you could pop back for a visit to thank him and/or commit retro-grand-patricide, couldn’t you? Not so fast. To make the logic blindingly obvious, if you kill your grandfather, then you won’t have been born, which means you couldn’t have killed your grandfather, which (logically) means that you will be born.
So, try as you might, you can’t kill your own grandfather, nor can you change history at all. The Terminator learned this the hard way, going back in time to prevent John Connor’s birth by killing his mother. When a human travels back in time to protect her, the two fall in love—and she becomes pregnant with … John Connor. Ta-da.
There’s no need for such finagling in The Time Traveler’s Wife. Since Henry DeTamble serves as his own time machine, there’s little chance of his preventing his own birth. Cf. rule No. 2.
All in all, the movie does pretty well, except for, you know, the fact that a genetic disorder really can’t cause spontaneous time travel, unless you live in the X-Men universe.